I know this may seem like a weird ask, but I have my reasons, and I hope you'll take the time to read this.

Currently, with just a sword, axe, pickaxe, shovel, and hoe slot in the toolbelt, it's perfectly alright, but could do with some improvements.

My proposal is that, when you have Deep Pockets 1, similar to how you get 4 additional toolbelt slots, you also get a row of slots specifically for silk touch tools. You'd have access to a keybind to switch between preferring fortune and preferring silk touch as well.
In addition to this, anything in your toolbelt slots (if they don't already do this, I haven't tested it yet) would get repaired by XP when they're enchanted with Mending, regardless of whether or not they're currently out, much like armor does.

When you get Deep Pockets 1, it'd add only 4 utility belt slots, just like usual. When you get Deep Pockets 3, you'd get another 4, making the maximum be 12. You could then reasonably stop there if you'd like to, but we could keep going.
Progressing onwards, we'd make it so Deep Pockets goes all the way to level 6, instead of just 3. At Deep Pockets 4, you'd get another row of inventory, but no utility belt slots yet. Deep Pockets 5 does the same. At Deep Pockets 6, however, you get the usual row of slots, making the total rows equate to that of a large chest, but you also get a whopping 8 utility belt slots, bringing the total from 12 to 20.

Levels 4 through 6 would be a bit more gimmicky, because not everybody would necessarily enjoy having access to that many utility belt slots. My proposal here is to limit the enchantment levels. Similar to how Goblin Traders offers Diamond Pickaxes with Level 6 Efficiency and Level 4 Fortune, you could use a system where the books can't be combined beyond Level 3, necessitating further exploration.

For those that'd prefer a more classic experience, they can opt to change a config that restricts the additional 4 utility belt slots from Deep Pockets 3 to also be Deep Pockets 6 only, as well as an option to force the silk touch toolbelt slots to be Deep Pockets 6 only.

Finally, there would be an option to just fully disable the levels beyond 3, but this wouldn't disable the silk slots or the extra rows outright, and if the option for this is set to remove the extra levels, config options would exist to just slap all of the slots into previous levels, putting the additional 4 into Deep Pockets 2, the additional 8 into DP3, and the silk slots into DP2, but they'd only work if the option to disable the additional levels was toggled, and would be set to true by default, meaning that disabling the extra levels would shift all the abilities down the enchantment levels once the option is changed.